Optical information processor and optical element

ABSTRACT

An optical disk device has an aperture of an objective lens in an incoming path of a beam from a semiconductor laser to an optical disk formed larger than an aperture in a return path from the optical disk or an aperture is varied in recording and in reproduction. This configuration improves recording/reproducing ability since light is focused on an optical disk with high numerical aperture. In addition, since reflected light from the optical disk is detected with low numerical aperture, margins for tilt and defocus are not reduced. Furthermore, since unnecessary signal components contained in the reflected light can be eliminated, a S/N (signal-to-noise ratio) of an information signal also increases. Thus, a high-performance optical disk device can be obtained. Alternatively, by varying the aperture of an objective lens in recording and in reproduction, an optical disk device in which recording density and recording quality are increased without deteriorating reproduction quality can be obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/436,847, filed Nov. 9, 1999 now U.S. Pat. No. 6,618,343.

FIELD OF THE INVENTION

The present invention relates to an optical information processor inwhich information is optically recorded on or reproduced from an opticaldisk, and to an optical element used in an optical pick-up.

BACKGROUND OF THE INVENTION

The operation of an optical head that is one of conventional opticalinformation processors is described with reference to FIGS. 18(a) and18(b). Light emitted from a semiconductor laser 18-1, an exemplary lightsource, passes through a hologram 18-5 as a separation element and thenis focused on an optical disk 18-2, as an exemplary informationrecording media, by an objective lens 18-3. After passing through theobjective lens, light reflected from the optical disk is diffracted bythe hologram and is incident onto first photodetectors 18-4-1 and18-4-2. An aperture in an optical path from the light source to theoptical disk (hereinafter referred to simply as an “incoming path”)through which light passes is determined by an objective lens holder18-6. A circular aperture is used in many cases. An aperture NAcorresponds to a diameter of light being incident onto the objectivelens. The diameter D satisfies the relationship of

D=2×f×NA,

wherein f represents a focal length of the objective lens. Since thefocal length f is constant, the size of the NA corresponds to the sizeof the diameter D. The aperture in an optical path from the optical diskto the photodetectors (hereinafter referred to simply as a “returnpath”) through which light reflected from the optical disk passes alsois determined by the objective lens holder 18-6. Therefore, theapertures in the incoming and return paths are equal.

A detection case of various signals is described. When the hologram isformed of a part of a Fresnel lens, it can be formed so that diffractedlight in one side is focused before reaching the photodetector 18-4-1and diffracted light in the other side is focused at a position behindthe photodetector 18-4-2 as shown in FIG. 18. As shown in a view seenfrom the A direction in FIG. 18, when the respective photodetectors18-4-1 and 18-4-2 are formed while being divided into three parts, afocus error signal FE in a SSD (spot size detection) system can bedetected from the calculation result of outputs from the respectivephotodetectors. The FE can be obtained from either:

FE=(18-4-1 b)−(18-4-2 b)  (1)

or

FE=((18-4-1 a)+(18-4-1 c)+(18-4-2 b))−((18-4-1 b)+(18-4-2 a)+(18-4-2c))  (2)

When a track direction on an optical disk coincides with the informationtrack direction shown in FIGS. 18(a) and 18(b), a far field pattern as adiffraction pattern produced by a track is formed at spots on thephotodetectors as shown in the view seen from the A direction.Therefore, the tracking error signal TE can be obtained from any one of:

TE=(18-4-1 a)−(18-4-1 c)  (3),

TE=(18-4-2 a)−(18-4-2 c)  (4),

and

TE=((18-4-1 a)+(18-4-2 c))−((18-4-1 c)+(18-4-2 a))  (5).

A data information signal RF of an optical disk can be obtained from allthe outputs of the photodetector 18-4-1 or 18-4-2, or the total outputsof the photodetectors 18-4-1 and 18-4-2.

FIG. 19 shows a configuration of an optical disk device in anotherconventional example using two laser beam sources that emit beams withdifferent wavelengths from each other. This optical disk device has twolaser beam sources 19-1 (emitting a beam with a wavelength λ1) and 19-2(emitting a beam with a wavelength λ2) that emit beams with differentwavelengths from each other. The laser beam 19-21 with a wavelength λ1(in the case of DVD or the like, λ1=660 nm) emitted from the laser beamsource 19-1 passes through a polarization hologram element 19-3.

This polarization hologram element is formed by forming a grating with adepth of d in a substrate made of an anisotropic material such aslithium niobate and filling groove parts of the grating with anisotropic material (with a refractive index of n1). Generally, given thephase difference φ between a beam passing through a groove portion and abeam passing between the groove portions, transmittance is representedby cos²(φ/2). When the substrate has refractive indexes of n1 and n2with respect to polarized lights parallel and perpendicular to thegrating grooves respectively, φ=0 holds with respect to the polarizedlight parallel to the grating grooves and therefore the transmittanceis 1. On the other hand, with respect to the polarized lightperpendicular to the grating grooves, φ=2π(n1−n2)d/λ. Therefore, whenthe depth d is set to obtain φ=π, the transmittance is 0, i.e. thepolarized light is totally diffracted.

Consequently, when considering the polarization direction of the beam19-21 emitted from the laser beam source 19-1 and groove orientation ofthe polarization hologram element 19-3, the laser beam 19-21 is allowedto pass through the element 19-3 without being diffracted. Thetransmitted light 19-22 is converted from linearly polarized light(S-wave) into circularly polarized light 19-23 by a ¼ wave plate 19-4,is reflected by a surface of a prism 19-5, and then is collimated intoparallel light 19-24 by a collimator lens 19-6. The parallel light 19-24enters an objective lens 19-8 mounted on a moving element 19-14 of anactuator via a mirror 19-7 for bending an optical path and is incidentonto a signal surface 19-9 of the optical disk.

In the case of recording on the signal surface, by increasing the powerfor emitting beams of the laser beam source 19-1 and modulating lightcorresponding to a recording signal, a required signal is recorded onthe signal surface 19-9.

The light 19-25 reflected from the signal surface 19-9 travels in theopposite direction to the incoming path. The light 19-25 is converted tolinearly polarized (P-wave) light 19-26 by the ¼ wave plate 19-4 andpasses through the polarization hologram element 19-3. In this case, dueto polarization dependability of the element 19-3 the light is branchedinto a positive first-order diffracted light 19-27 and a negativefirst-order diffracted light 19-28 whose symmetry axis is theincident-light axis. The lights 19-27 and 19-28 are incident ontodetection surfaces on photodetectors 19-10 provided adjacently to thelaser beam source 19-1. Thus, a control signal and a reproduction signalare obtained to reproduce information.

On the other hand, a laser beam 19-29 emitted from the semiconductorlaser beam source 19-2 emitting a beam with the other wavelength λ2 (inthe case of CD or the like, 790 nm) passes through a hologram element19-11 to be diffracted and branched into three beams (a positivefirst-order diffracted light, a negative first-order diffracted light, azeroth-order light). The three beams pass through the prism 19-5 whilebeing limited by an aperture element 19-12 provided on a light-incidentsurface of the prism 19-5 and are collimated by the collimator lens 19-6into convergent light 19-30. Then, the convergent light passes throughthe objective lens 8 via a mirror 19-7 for bending an optical path, thusbeing incident onto a signal surface 19-15 of an optical disk whosesubstrate has a different thickness from that when using the laser beamsource 19-1. In this case, the diffracted light caused by the hologramelement 19-11 is allocated to three spots on the signal surface and isused for the detection of a tracking control signal and a reproductionsignal by a so-called three-beam tracking method. Light 19-31 reflectedfrom the signal surface 19-15 is diffracted by the hologram element19-11 via the mirror 19-7, the collimator lens 19-6, and the prism 19-5.Then, the diffracted light is incident onto detection surfaces ofphotodetectors 19-16, thus detecting signals to reproduce information.The objective lens 19-8 is designed to have a shape that enablesaberration to be minimum by optimally designing the aperture and theoptical system for respective disks having a substrate thickness of 0.6mm for the beam with the wavelength λ1 and having a substrate thicknessof 1.2 mm for the beam with the wavelength λ2. In other words, withrespect to the beam with the wavelength λ2, the aperture is limited bythe aperture element 19-12 to form an optimum aperture.

With increase in density of the data information, further improvement inrecording and reproducing ability is required in optical disk devices.Generally, in order to record and reproduce signals with higher density,a focusing spot on a disk is reduced in size. That is, it is conceivablethat the wavelength of light emitted from a light source is shortened orNA of an objective lens is increased. However, in general-purposeoptical disk devices used in a general office or at home, an availableshort-wavelength light source is a semiconductor laser emitting a redbeam with 660 nm at present. A semiconductor laser emitting a beam witha shorter wavelength than that lacks in reliability and therefore it isdifficult to use it for recording purpose in the present situation. Whenthe NA of the objective lens is increased (i.e. when the aperture of anobjective-lens holder is enlarged), recording/reproducingcharacteristics are improved in part. However, margins for tilt anddefocus are reduced greatly, which has been a problem. In one or moreembodiments, it is a first object of the present invention to provide anoptical disk device in which excellent recording and reproduction can beperformed on an optical disk with higher density and the margins are notreduced at the same time.

On the other hand, there have been the following three problems in aconventional optical disk device using the two laser beam sources shownin FIG. 19.

Firstly, when the lens is shifted in a track direction of an opticaldisk, the relative position of the lens and the aperture element varies,thus causing asymmetry in the aperture. Consequently, aberration (mainlyspherical aberration and coma aberration) is increased, thusdeteriorating signal quality considerably.

Secondly, similarly when the lens is shifted, the relative position ofthe lens and the hologram varies and therefore unbalance in quantity oflights divided by the hologram and distributed to photodetectors occurs,thus causing offset of a signal due to DC components, which is notpreferable in tracking control.

Thirdly, generally due to refractive index variance of an objective lensor a collimator lens, when the wavelength mode of a beam emitted from alaser beam source is changed by power modulation for recording andreproduction, momentary axial aberration (i.e. chromatic aberration)occurs. Consequently, a relative position error (defocus) between a lensand a signal surface is caused. In order to prevent this, any chromaticaberration compensation element is required. FIGS. 20(a) and 20(b) showa cross-sectional structural view and a plane view of a conventionallyproposed chromatic-aberration compensation element 20-160 (seeJP-A-6-82725 about the details). The chromatic-aberration compensationelement 20-160 is formed of a glass plate having a refractive index n inwhich a concentric stepped structure 20-150 is formed. In the figure,the phase of a beam with a wavelength λ that passes through theconcentric stepped structural portion having a stepped depth trepresented by:

t=λ/(n−1)

is shifted for 2π between adjacent stepped portions. However, withrespect to undulation, the same wavefront is formed. On the other hand,when the wavelength is shifted from λ, the phase of the light is shiftedslightly between adjacent stepped portions. However, since this steppedstructure is formed in a concentric shape, an almost spherical wave isgenerated in a direction canceling axial aberration caused by thechromatic aberration. Thus, the aberration can be compensated bycombining this element with a lens.

In order to solve all the three problems of the optical disk deviceshown in FIG. 19, it is preferable to mount all the components describedabove (the aperture element, the hologram element, the chromaticaberration compensation plate) on a moving element. However, when allthese elements are mounted, the moving element becomes very heavy and inaddition, it is difficult to keep the actuator in balance. Further, asthe weight of the actuator increases, more energy is required for itsoperation, thus causing problem of high power consumption. In addition,since all the elements must be positioned accurately with respect to thecenter of lenses (the center of optical axes), highly accurate assemblyprocesses are necessary, thus decreasing mass-productiveness. In one ormore embodiments, a second object of the present invention is directedto solve these problems.

SUMMARY OF THE INVENTION

In order to attain the first object, one or more embodiments of thepresent invention provide a configuration in which NA of an objectivelens positioned in an incoming optical path from a semiconductor laserto an optical disk is designed to be larger than that in a return pathfrom the optical disk or NA is varied in recording information and inreproducing information.

This configuration enables recording/reproducing ability to be improvedsince light is focused onto an optical disk with high NA. In addition,since reflected light from the optical disk is detected with low NA,margins for tilt and defocus are not reduced. Furthermore, sinceunnecessary signal components contained in the reflected light can beeliminated, a S/N (signal-to-noise ratio) of an information signal alsoincreases. Thus, a high-performance optical disk device can be obtained.Alternatively, by varying the aperture of an objective lens in recordinginformation and in reproducing information, an optical disk device inwhich recording density and recording quality are increased withoutdeteriorating reproduction quality can be obtained.

Further, one or more embodiments of the present invention employ thefollowing means to attain the second object. Embodiments of the presentinvention are characterized by an optical element in which a thin filmfor varying an aperture area corresponding to each of two wavelengths(wavelengths λ1 and λ2; λ1<λ2) of light is formed on one of two glassplates of a polarization hologram element. The polarization hologramelement is formed by sandwiching a diffraction grating made of abirefringent material and a wave film with an optical thickness of(N1+¼) λ1, wherein N represents a natural number, between the glassplates. Embodiments of the present invention are characterized by anoptical element having a structure having a plurality of concentricstepped portions on the other glass plate. In one or more embodiments,the present invention is characterized in that wavelengths λ1 and λ2 oftwo types of lights passing through an optical element satisfy therelationship of (N1+¼) λ1≈N2×λ2, wherein N1 and N2 represent naturalnumbers. Embodiments of the present invention are characterized in thatan optical element using a wave film with an optical thickness of (N1+⅕)λ1 instead of the wave film with an optical thickness of (N1+¼) λ1 ismounted on an actuator.

According to the aforementioned configurations of the present invention,the following excellent effects can be obtained. By varying the aperturein an incoming path and in a return path in recording information on orreproducing information from an optical disk, excellent recording andreproduction are performed by obtaining a spot focused with high NA inthe incoming path, and in the return path, crosstalk compositions,intersymbol interference compositions, high aberration compositionscontained a lot in reflected light from the optical disk that passesthrough high NA portions are eliminated by applying low NA, thusenabling high-quality signal reproduction. In addition, the margins fordefocus and tilt are not reduced.

Furthermore, by using a diffraction grating as an aperture element andcombining with another element to form one component, excellent effectssuch as reduction in size, stabilization, reduction in cost, and thelike can be obtained.

By leading at least a part of light outside the aperture in the returnpath to second photodetectors and calculating with outputs from firstphotodetectors, intersymbol interference compositions and crosstalkcompositions can be canceled out, thus providing an excellent effect inwhich further excellent information signals can be obtained.

In addition, by designing the aperture of an aperture element to bevariable, an optimum aperture can be set for respective optical disksand therefore an excellent effect enabling excellent signal reproductioncontinuously can be provided, thus obtaining an optical disk device inwhich recording density and recording quality are improved withoutdeteriorating reproduction quality.

Moreover, according to the aforementioned configurations, a movingelement is not greatly increased in weight, thus suppressing theincrease in power consumption. In addition, precise positioning betweenrespective elements is not required, thus facilitating the assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b), and 1(c) show structural views showing an opticalinformation processing method according to a first embodiment of thepresent invention.

FIG. 2 is an analytical example according to the first embodiment of thepresent invention.

FIGS. 3(a), 3(b), 3(c) and 3(d) show structural examples of an apertureelement according to the first embodiment of the present invention.

FIGS. 4(a), 4(b), and 4(c) show structural views showing an opticalinformation processing method according to a second embodiment of thepresent invention.

FIG. 5 is an analytical example according to the second embodiment ofthe present invention.

FIG. 6 is a structural example of an aperture element according to thesecond embodiment of the present invention.

FIGS. 7(a), 7(b), and 7(c) show structural views showing an opticalinformation processing method according to a third embodiment of thepresent invention.

FIG. 8 is an analytical example according to the third embodiment of thepresent invention.

FIG. 9 is a structural example of an aperture element according to thethird embodiment of the present invention.

FIGS. 10(a), 10(b), and 10(c) show structural views showing anotheroptical information processing method according to the third embodimentof the present invention.

FIGS. 11(a), 11(b), and 11(c) show structural views showing an opticalinformation processing method according to a fourth embodiment of thepresent invention.

FIGS. 12(a), 12(b), and 12(c) show structural views showing an opticalinformation processing method according to a fifth embodiment of thepresent invention.

FIGS. 13(a), 13(b), and 13(c) show structural views showing an opticalinformation processing method according to a sixth embodiment of thepresent invention.

FIG. 14 is a structural example of an aperture element according to thesixth embodiment of the present invention.

FIG. 15 is a cross-sectional structural view of an aperture elementaccording to a seventh embodiment of the present invention.

FIG. 16 shows a configuration of an optical disk device using anaperture element according to the seventh embodiment of the presentinvention.

FIG. 17 is a cross-sectional structural view of an aperture elementaccording to an eighth embodiment of the present invention.

FIGS. 18(a) and 18(b) show structural views of a conventional opticalinformation processing method.

FIG. 19 shows a configuration of a conventional optical disk device.

FIGS. 20(a) and 20(b) show a cross sectional view and a plane view of anelement having a concentric stepped structure as a conventional meansfor compensating chromatic aberration.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

FIGS. 1(a), 1(b), and 1(c) show structural examples of an opticalinformation processing method according to a first embodiment of thepresent invention. The description of the same points, such as adetection principle of FE, TE, RF signals as those in the conventionalexample, shown in FIGS. 18(a) and 18(b) is omitted. In the conventionalexample, the NA in an incoming path and the NA in a return path aredetermined by the objective lens holder 18-6 and are equal. In thepresent embodiment, an aperture is determined by an objective lensholder 1-6, a λ/4 plate 1-7 that is a component of an aperture element,and a diffraction grating 1-8. The diffracting grating 1-8 is providedwith a grating at the portion indicated with hatching in the figure.

The operation of the aperture element is described as follows. The λ/4plate 1-7 has a function of providing a phase difference of λ/4 toincident light. The diffraction grating 1-8 is a grating having concaveand convex portions made of an anisotropic material (with a refractiveindex of, for example, n1 and n2) such as, for example, lithium niobateand is formed by filling the concave portions with an isotropic materialwith a refractive index (for example, n1) equal to any one of the tworefractive indexes of the anisotropic material. When linearly polarizedlight is incident onto the diffraction grating 1-8, all the incidentlight passes through the diffraction grating 1-8, since with respect tolight with a polarization direction in which the refractive index of n1of the anisotropic material is effective, the diffraction grating 1-8does not function (i.e. this case is equal to the case where nodiffraction grating is provided). On the other hand, with respect toincident light with a polarization direction orthogonal to theabove-mentioned polarization direction, the diffraction grating 1-8functions to diffract the incident light. In FIGS. 1(a), 1(b), and 1(c),the polarization direction of a beam emitted from the semiconductorlaser 1-1 corresponds to the polarization direction of a beam for whichthe grating of the diffraction grating 1-8 does not function. Therefore,all the incident light onto the diffraction grating 1-8 from thesemiconductor laser 1-1 passes through the diffraction grating 1-8regardless of the existence of the grating. Thus, the aperture NA1 inthe incoming path is not determined by the aperture element, but isdetermined by the objective lens holder 1-6. On the other hand,reflected light from the optical disk passes through the λ/4 plate 1-7twice, i.e. in the incoming and return paths and therefore itspolarization direction is orthogonal to that of a beam emitted from thesemiconductor laser 1-1. Consequently, since the grating of thediffraction grating 1-8 is effective, light passes through the centralpart (the area without hatching in the figure) of the diffractiongrating 1-8 to reach first photodetectors. However, the grating portion(indicated with hatching in the figure) diffracts light as shown in thefigure and therefore the light does not reach the first photodetectors.The aperture NA2 in the return path is determined not by the objectivelens holder 1-6 but by the diffraction grating 1-8.

As can be seen from FIGS. 1(a), 1(b), and 1(c), NA1 is greater than NA2.For facilitating the description, suppose that the NA2 is equal to theNA of the aperture element in the conventional example. In this case,when comparing the present embodiment with the conventional example, theaperture in the return path is equal but the aperture in the incomingpath is larger in the present invention. This provides the followingeffects for the present invention, which are not provided by theconventional example.

(1) A spot size on an optical disk is proportional to

λ/NA,

wherein λ represents a wavelength. Therefore, in the present inventionin which the aperture NA1 for the incident light onto the optical diskis greater than the NA in the conventional example, a small spot sizecan be obtain on the optical disk, thus improving recording sensitivity,recording quality, and resolution of reproduction signals and reducingcrosstalk and intersymbol interference.

(2) The decrease in signal level caused by defocus is proportional tothe square of NA and that caused by tilt is proportional to the cube ofNA. In the present invention, since the aperture in the return path isNA2 that is equal to the conventional NA, the margins for defocus andtilt are equivalent to those in the conventional example. Therefore,although the aperture in the incoming path is larger, the margins arenot reduced in the present invention.

(3) The crosstalk components due to adjacent tracks, intersymbolinterference components due to sequential two signals, and aberrationcomponents due to defocus and tilt are contained a lot in the area(surroundings of the aperture) with high NA for the reflected light fromthe disk. In the present embodiment, the aperture in the return path isset to be smaller than that in the incoming path. Consequently, lightcontaining a lot of crosstalk components, intersymbol interferencecomponents, and aberration components can be eliminated, thus obtainingexcellent reproduction characteristics.

FIG. 2 shows an analytical example of reproduction characteristicsaccording to the present embodiment. A wavelength λ of a beam emittedfrom an optical head reproducing random data comprising marks betweenabout 0.4 μm and 2 μm as information signals on an optical disk having atrack pitch of 0.6 μm is supposed to be 660 nm. FIG. 2 shows jitter inreproduction signals using NAs in the incoming and return paths as aparameter. In FIG. 2, the jitter is shown by contour lines at intervalsof 0.1%. In the case of a conventional example, when both the NAs in theincoming and return paths are set to be 0.6 as a general aperture, thereproduction-signal jitter is about 4.05%. On the contrary, when NA1 inthe incoming path is 0.63 and NA2 in the return path is 0.6 as anexample of the present embodiment, the reproduction-signal jitter is3.0%, which shows 1.05% improvement compared to the conventionalexample. As a conventional example, when both the NAs in the incomingand return paths are set to be 0.63, the reproduction-signal jitter isabout 3.07%, which falls 0.07% short of that in the present invention.In addition, since the NA in the return path also is 0.63, the defocusmargin and the tilt margin are reduced by about 9% and about 14%respectively compared to those in the present embodiment.

In the case of the supposed optical disk in the above-describedanalysis, when the NA2 in the return path is set to be a general valueof 0.6, the jitter decreases gradually as the NA1 in the incoming pathincreases. However, the variation in jitter becomes small, when the NA1is in the neighborhood of 0.63 to 0.67. In order to reduce the influenceof the variation in the NA1, it is desirable to set the NA1 to bebetween 0.63 and 0.67. When priority is given to the defocus- andtilt-margin expansion by reducing the NA2 in the return path, it isfound that the jitter does not deteriorate even when the NA2 in thereturn path is 0.54 or less in the case where the NA1 in the incomingpath is 0.60. An optimum NA ratio varies depending on the purpose suchas, for example, giving priority to jitter, giving priority to margins,allowing the jitter and margins to be compatible. However, the optimumcondition can be found when the NA ratio is set substantially in a rangeof

1<NA 1/NA 2<1.2.

FIGS. 3(a), 3(b), 3(c), and 3(d) show examples of an aperture element inthe present embodiment. FIG. 3(a) is a view of the aperture element withthe configuration shown in FIGS. 1(a), 1(b), and 1(c) that is seen fromthe side of the objective lens 1-3. The portion indicated with hatchingis the lens holder 1-6 and its aperture corresponds to NA1 in theincoming path. Concentric portions inside the hatching portion denotethe grating of the diffraction grating 1-8, and its aperture correspondsto NA2 in the return path. In FIG. 3(a), both the apertures in theincoming and return paths are formed in circular shapes. However, theshapes of the apertures are not limited to the circular shapes. In thecase of the circular shape, an advantage of facilitating the processingand formation of apertures of the lens holder 1-6 and the like can beobtained. Generally, however, an optimum NA in a radial direction on anoptical disk is different from that in a tangential direction in somecases. In this case, the aperture with an elliptical shape is preferredto that with a circular shape. FIG. 3(b) shows the case where theaperture of the objective lens holder 1-6 has an elliptical shape withhigh NA in the tangential direction. The direction in which the aperturehas high NA is not limited to the tangential direction and may be theradial direction. Further, there may be the case where the apertureformed by the diffraction grating 1-8 in the return path has anelliptical shape. The excellent effect of the present embodiment can beobtained by a smaller aperture in the return path compared to that inthe incoming path. Therefore, generally the shape of the aperture is nota problem. Even when the aperture in the return path has a square shapeas shown in FIG. 3(c), the effect of the invention is not reduced. FIG.3(d) shows an example of aperture in the return path formed of fourcircular apertures, which has the excellent effect of the presentinvention. In addition, since light passing through the central portionof the aperture in the incoming path also is eliminated in the returnpath, light containing a lot of DC components is eliminated. Therefore,the example also has an advantage of improving a modulation factor ofdata information signal RF. As described above, in one or moreembodiments, the present invention is characterized by setting theaperture in the return path to be smaller than that in the incoming pathand therefore the shape of the aperture is not particularly a problem.

Second Embodiment

FIGS. 4(a), 4(b), and 4(c) show structural examples according to asecond embodiment of the present invention when the apertures in theincoming and return paths are varied only in the radial direction. Thedescription of the same parts as those in the configuration shown inFIGS. 1(a), 1(b), and 1(c) is omitted. As shown in FIGS. 4(a), 4(b), and4(c), the aperture formed by an objective lens holder 4-1 in theincoming path and the aperture formed by a diffraction grating 4-2 inthe return path are set to be equal in the tangential direction. On thecontrary, the aperture NA2 (R) in the return path is set to be smallerthan the aperture NA1 (R) in the incoming path in the radial directionshown in FIG. 4(b). In an optical disk system that is affected less byintersymbol interference in the tangential direction, the followingadvantages are obtained compared to an optical disk system having theconfiguration shown in FIGS. 1(a), 1(b), and 1(c).

(1) Since the aperture NA in the return path is smaller than that in theincoming path in the radial direction, not only the decrease incrosstalk in adjacent tracks and the improvement in recordingsensitivity are achieved, but also crosstalk compositions and highaberration portions contained a lot in reflected light from a disk thatpasses through high NA portions can be eliminated. Consequently,excellent reproduction signals can be obtained, and in addition themargins are not reduced by defocus and tilt.

(2) Since the aperture in the incoming path is equal to that in thereturn path in the tangential direction, the loss in quantity of lightdue to reduction in NA in the return path can be reduced, thus obtainingreproduction signals with a high S/N.

FIG. 5 shows an analytical example of reproduction characteristics whenthe apertures in the incoming and return paths are set to be 0.60 in thetangential direction and the apertures in the incoming and return pathsare varied only in the radial direction. The parameter of the opticaldisk is the same as in the embodiment 1. When both the apertures in theincoming and return paths are 0.63 in the radial direction, jitter isabout 3.54%. However, it can be found that when the aperture in theincoming path is fixed to 0.63 and the aperture in the return path isdecreased from 0.63, the jitter is improved to 3.5% or less in the casewhere the aperture in the return path is between 0.585 and 0.62. On thecontrary, when the aperture in the return path is fixed to 0.60 and theaperture in the incoming path is increased from 0.60, the minimum jitteris obtained in the case where the aperture in the incoming path is about0.64 and the jitter is improved in the case where the aperture in theincoming path is between about 0.60 and 0.72. An optimum NA ratio variesdepending on the purpose such as, for example, giving priority tojitter, giving priority to margins, allowing the jitter and margins tobe compatible. However, the optimum condition can be found when the NAratio is set substantially in the range of

1<NA 1(R)/NA 2(R)<1.2.

FIG. 6 shows a structural example of an aperture element in the presentembodiment. The hatching portion shows an objective lens holder 4-1 withan aperture in the incoming path having an elliptical shape with itsmajor axis in the radial direction. The circular aperture inscribed inthe elliptical aperture is an aperture formed by a diffraction grating4-2 in the return path, and the portion indicated with horizontal linesin the figure is a grating. In the present configuration, since thecircular aperture is inscribed in the elliptical aperture in thetangential direction, the aperture in the incoming path is equal to thatin the return path in the tangential direction, and the aperture NA1(R)in the incoming path is larger than the aperture NA2(R) in the returnpath only in the radial direction. FIG. 6 illustrates the combination ofan ellipse and a circle as shapes of apertures. However, the shapes ofthe apertures are not particularly limited as described in the firstembodiment and as shown in FIGS. 3(a), 3(b), 3(c), and 3(d).

Third Embodiment

FIGS. 7(a), 7(b), and 7(c) show structural examples according to a thirdembodiment of the present invention when apertures in incoming andreturn paths are varied only in the tangential direction. Thedescription of the same parts as those in the configurations shown inFIGS. 1(a), 1(b), and 1(c) and 4(a), 4(b), and 4(c) is omitted. As shownin FIG. 7(b), the aperture formed by an objective lens holder 7-1 in theincoming path and the aperture formed by a diffraction grating 7-2 inthe return path are set to be equal to each other in the radialdirection. On the contrary, in the tangential direction, an apertureNA2(T) in the return path is set to be smaller than an aperture NA1(T)in the incoming path. In an optical disk system that is affected less bycrosstalk in the radial direction, the following advantages are obtainedcompared to the configuration shown in FIGS. 1(a), 1(b), and 1(c).

(1) Since the aperture NA in the return path is smaller than that in theincoming path in the tangential direction, not only the decrease inintersymbol interference and the improvement in resolution and recordingsensitivity are achieved, but also intersymbol interference compositionsand high aberration portions contained a lot in reflected light from adisk that passes through high NA portions can be eliminated.Consequently, excellent reproduction signals can be obtained, and inaddition the margins are not reduced by defocus and tilt.

(2) Since the aperture in the incoming path is equal to that in thereturn path in the radial direction, the loss in quantity of light dueto reduction in the NA in the return path can be reduced, thus obtainingreproduction signals with a high S/N.

FIG. 8 shows an analytical example of reproduction characteristics whenboth the apertures in the incoming and return paths are set to be 0.60in the radial direction and the apertures in the incoming and returnpaths are varied only in the tangential direction. The parameter of theoptical disk is the same as in the first and second embodiments. Whenboth the apertures in the incoming and return paths are 0.66 in thetangential direction, jitter is about 3.2%. However, it can be foundthat when the aperture in the incoming path is fixed to 0.66 and theaperture in the return path is decreased from 0.66, the jitter isimproved to 3.2% or less in the case where the aperture in the returnpath is between 0.57 and 0.66. An optimum NA ratio varies depending onthe purpose such as, for example, giving priority to jitter, givingpriority to margins, allowing the jitter and margins to be compatible.However, the optimum condition can be found when the NA ratio is setsubstantially in the range of

1<NA 1(T)/NA 2(T)<1.2.

FIG. 9 shows a structural example of an aperture element according tothe third embodiment of the present invention. The hatching portionshows an objective lens holder 7-1 with an aperture in the incoming pathhaving an elliptical shape with its major axis in the tangentialdirection. A circular aperture inscribed in the elliptical aperture isthe aperture formed by a diffraction grating 7-2 in the return path, andthe portion indicated with vertical lines in the figure is a grating. Inthe present configuration, since the circular aperture is inscribed inthe elliptical aperture in the radial direction, the aperture in theincoming path is equal to that in the return path in the radialdirection, and the aperture NA1(T) in the incoming path is larger thanthe aperture NA2(T) in the return path only in the tangential direction.FIG. 9 illustrates the combination of an ellipse and a circle as shapesof apertures. However, the shapes of the apertures are not particularlylimited as described in the first embodiment and as shown in FIGS. 3(a),3(b), 3(c), and 3(d).

FIGS. 10(a), 10(b), and 10(c) show structural examples of the presentembodiment using a λ/4 plate 1-7 and a polarized beam splitter(hereinafter referred to as “PBS”) 10-1 as aperture elements. In apolarization film of the PBS 10-1, incident light is transmitted orreflected depending on its polarization direction. Therefore, the PBS10-1 has the same function as that of the diffraction grating 7-2. As anaperture element, the PBS is desirable for eliminating light passingthrough the high NA portions with a high quenching ratio, and thediffraction grating is desirable for achieving the reduction in size andthickness. As another configuration of the aperture element, it ispossible to combine, for example, a λ/4 plate and liquid crystal. Anoptimum configuration may be selected depending on the intended use.

Fourth Embodiment

A configuration according to a fourth embodiment of the presentinvention is described with reference to FIGS. 11(a), 11(b), and 11(c).The description of the same parts as in the first to third embodimentsis omitted. A semiconductor laser 11-1 as a light source and firstphotodetectors 11-2-1 and 11-2-2 are combined with a base 11-3 to formone component. A beam emitted from the semiconductor laser 11-1 isincident onto an optical disk 1-2 via a diffraction grating 11-5, a λ/4plate 1-7, and an objective lens holder 11-4 that form one componentwith the objective lens 1-3. Reflected light from the optical disk 1-2is diffracted by the diffraction grating and a part of the diffractedlight is incident onto the first photodetectors. In the presentembodiment of the invention, the diffraction grating 11-5 has both thefunctions of the diffracting grating 1-8 and the hologram 1-5 as aseparation element in FIGS. 1(a), 1(b), and 1(c). The portion indicatedwith hatching in the diffraction grating 11-5 has a function ofdiffracting unwanted light outside the first photodetectors, and thecentral portion of the diffraction grating 11-5 has a function of thehologram 1-5. The present embodiment provides the following excellenteffects.

(1) The diffraction grating also has the function of a hologram as aseparation element, thus enabling the reduction in number of components,in size, and in cost.

(2) The aperture elements and the objective lens are combined to formone component, thus reducing the influence by the movement of theobjective lens and the like.

(3) The light source and the first photodetectors are combined to formone component, thus achieving the size reduction and stabilization of anoptical system.

It is not necessary to satisfy the above-mentioned effects (1), (2), and(3) at the same time. Even when one of the above-mentioned effects issatisfied individually according to the convenience in the configurationof the optical system or the like, the same individual effect can beobtained.

In FIG. 11(c), exactly speaking, only the base 11-3 can be seen. Inorder to facilitate the description, the view is shown in a manner ofseeing through the semiconductor laser and the photodetectors.

FIGS. 11(a), 11(b), and 11(c) show a configuration in which theapertures in incoming and return paths are varied from each other bothin the radial and tangential directions. However, needless to mention,the same effect can be obtained even when the apertures in the incomingand return paths are equal either in the radial or tangential direction.

Fifth Embodiment

A configuration according to a fifth embodiment of the present inventionis described with reference to FIGS. 12(a), 12(b), and 12(c). Thedescription of the same parts as in the fourth embodiment is omitted. Inthe present embodiment, four elements as second photodetectors areprovided on a base 12-1. Incident light onto a grating portion of adiffraction grating 12-3 out of reflected light from an optical disk isdiffracted to be led to the second photodetectors. The light led to thesecond photodetectors contains a lot of information about intersymbolinterference in the tangential direction and information about crosstalkin the radial direction. In the configurations of the first to fourthembodiments, excellent information signals were detected by eliminatingthis light. In the present embodiment, information signals are obtainedby calculating outputs from the first photodetectors and outputs fromthe second photodetectors. Even when light passing through the high NAportions in an aperture is eliminated, the intersymbol interferencecomponents and crosstalk components cannot be eliminated from the lightincident onto the first photodetectors completely. By subtracting theoutputs from the second photodetectors containing a lot of intersymbolinterference components and crosstalk components from the outputs fromthe first photodetectors, the intersymbol interference components andcrosstalk components contained in the outputs from the firstphotodetectors can be cancelled out, thus further improving informationsignal quality. In FIG. 12, the light passing through the high NAportions in both the tangential and radial directions is led to thesecond photodetectors. However, the light only in either one of thedirections may be led to the second photodetectors as required. In asystem affected slightly by the intersymbol interference, even in theconfiguration in which the light passing through the high NA portiononly in the radial direction is led to the second photodetectors,information signal quality can be improved. The same is applied to thecase where the light passing through the high NA portions only in thetangential direction is led to the second photodetectors.

Sixth Embodiment

A configuration according to a sixth embodiment of the present inventionis described with reference to FIGS. 13(a), 13(b), and 13(c). Thedescription of the same parts as in the first embodiment is omitted. Inthis embodiment of the present invention, an aperture element is formedof a λ/4 plate 1-7, a liquid crystal element 13-1, and a driving circuit13-2 of the liquid crystal element 13-1. The liquid crystal element 13-1functions to diffract a predetermined portion of reflected light from anoptical disk outside first photodetectors as in the diffraction grating1-8. However, in the liquid crystal element of the present embodiment,the region in which light is diffracted can be varied. FIG. 14 shows astructural example of the liquid crystal element. In FIG. 14, theportion indicated with hatching is the liquid crystal 14-1 that issandwiched between transparent electrodes 14-2 and 14-3 in the verticaldirection. The respective transparent electrodes are provided withtranslucent substrates 14-4 and 14-5 formed of glass or the like. A sideface of the liquid crystal 14-1 is sealed with a sealing material 14-6.When voltage is applied to the upper and lower transparent electrodes bya driving circuit 14-7, portions of the liquid crystal that aresandwiched between respective periodic structure portions of the uppertransparent electrode 14-2 show anisotropy corresponding to the periodof the transparent electrode and thus functions as an anisotropicdiffraction grating. In the present embodiment, the upper transparentelectrode 14-2 is divided into three regions and is formed so thatvoltage can be applied to respective three regions individually by thedriving circuit 14-7 through switching a switching circuit 14-8. In FIG.14, only (a) of the switching circuit 14-8 is ON, and therefore only oneregion in an outer side of the transparent electrode 14-2 functions as adiffraction grating. Depending on the state of the switching circuit14-8, the region functioning as a diffraction grating can be changed. Inthe present embodiment, the transparent electrode 14-2 was divided intothree regions, but of course the number of regions to be divided ischanged as required. Since the region functioning as a diffractiongrating in an aperture element is changed by using liquid crystal tomake the aperture in the return path variable, the present embodimentcan provide an excellent effect in which an optimum aperture can be setaccording to an optical disk. When performing information recording onand information reproduction from a plurality of optical disks havingdifferent track pitches and bit pitches from one another, it also ispossible to learn an optimum aperture for each optical diskautomatically.

In the present embodiment, the aperture in the return path is allowed tobe variable by the liquid crystal element, but of course the same canapply to the aperture in the incoming path. It also is possible to allowboth the apertures in the incoming and return paths to be variable toperform recording and reproduction as required. In the presentembodiment, only the configuration corresponding to the first embodimentis shown. However, the configuration is not limited to this, and theaperture elements in any configurations of the second to fifthembodiments are allowed to be variable.

Seventh Embodiment

FIG. 15 shows a cross-sectional configuration of an aperture element15-41 according to a seventh embodiment of the present invention. In thepresent embodiment, an aperture element is formed of a polarizationhologram formed of a ¼ wave plate and a diffraction grating made of abirefringent material.

A film 15-45 made of birefringent resin provided adjacently to aadhesion layer 15-44 has an optical thickness corresponding to {fraction(5/4)} wavelengths with respect to a beam with a wavelength λ1 (in thiscase, 660 nm) with its refractive index, thickness, birefringenceorientation with respect to a polarization direction being optimized.The optical thickness mentioned above corresponds to almost 1 wavelengthwith respect to a beam with a wavelength λ2 (in this case, 790 nm)emitted from a light source. Therefore, with respect to the beam withthe wavelength λ1, linearly polarized light passes through theabove-mentioned polarization hologram layer without being diffracted andthe light that has been reflected by a reflection surface and isincident from the opposite direction is totally diffracted by thepolarization hologram layer. On the other hand, with respect to the beamwith the wavelength λ2, a plane of polarization is not varied andtherefore the beam is not diffracted even when passing through theelement both in the incoming and return paths.

In the present embodiment, the optical thickness of the film 45corresponds to {fraction (5/4)} wavelengths with respect to the beamwith a wavelength of 660 nm. However, when the wavelengths are λ1(nm)and λ2(nm), generally the wave plate is designed according to thefollowing condition,

(N 1+¼)λ1≈N 2×λ2,

wherein N1 and N2 represent arbitrary natural numbers.

On the other glass plate 15-47, a color separation film 15-48 thattransmits the beam with the wavelength λ1 and shields the beam with thewavelength λ2 is formed. Furthermore, a phase adjustment film 15-49 forcompensating the phase difference between lights passing through regionsA and B is formed on the glass plate 15-47. Thus, the beam with thewavelength λ1 passes through both the regions A and B, and the beam withthe wavelength λ2 passes through only the region B. In other words, theaperture is restricted.

As another embodiment, a {fraction (6/5)} wave film 15-51 may be usedinstead of the {fraction (5/4)} wave film. In this case, light thatreturns to the laser beam source emitting the beam with the wavelengthλ1 is caused intentionally by preventing the total diffraction of thebeam with the wavelength λ1 and thus the stability of the laser can beimproved.

FIG. 16 shows a configuration of an optical disk device using theaperture element according to the seventh embodiment of the presentinvention.

A laser beam 16-21 with a wavelength of 660 nm emitted from asemiconductor laser beam source 16-1 is reflected by a surface of aprism 16-5 and is collimated by a collimator lens 16-6 into parallellight 16-24. Then the parallel light 16-24 passes through an apertureelement 16-41 according to the seventh embodiment of the presentinvention without being diffracted via a mirror 16-7 for binding anoptical path and is converted from linearly polarized light intocircularly polarized light via a {fraction (5/4)} wave film 16-43comprised in the aperture element 16-41. Thus, a beam with a diameter ofD1 is incident onto an objective lens 16-8. The objective lens 16-8focuses the beam with a diameter of D1, and then the focused beam isincident onto a signal surface 16-9 on a disk. When information isrecorded on the signal surface, required signals are recorded on thesignal surface 16-9 by increasing the power for emitting beams of thelaser beam source 16-1 and applying modulation corresponding torecording signals.

Light reflected by the signal surface 16-9 travels in the oppositedirection to that in the incoming path, is converted to linearlypolarized light in the direction orthogonal to the incoming path, andthen is incident onto a polarization hologram portion in the element16-41. The incident light is branched into diffracted lights 16-27′,16-27 a, 16-28′, and 16-28 b whose symmetry axis is an optical axis ofthe incident light, depending on its polarization dependability.Diffracted light of a beam with a diameter of D2 out of theabove-mentioned diffracted lights is incident on detection surfaces onphotodetectors 16-10 provided adjacently to the light source via themirror 16-7, the collimator lens 16-6, and the prism 16-5. Thus, controlsignals and reproduction signals are obtained and thus reproduction isperformed. The diffracted lights 16-27 a and 16-28 b that do notcontained in the diameter D2 of the beam are not led to thephotodetectors and thus are not used for reproduction signals.

When the aperture of the objective lens mentioned above is set to be

D 1>D 2,

wherein D1 and D2 represent an aperture of the objective lens inrecording and an aperture of the objective lens in reproductionrespectively, NA is proportional to the aperture in the same objectivelens and therefore the relationship of

NA 1>NA 2

is satisfied, wherein NA1 and NA2 represent NA of the objective lens inrecording and NA of the objective lens in reproduction, respectively.Consequently, NA is varied in recording and in reproduction.

In the present embodiment, the NA in recording is larger, but on thecontrary, it also is easy to make the NA in reproduction larger.

A laser beam with a wavelength of 780 nm emitted from another lightsource 16-2 is diffracted and branched to three beams (positivefirst-order diffracted light, negative first-order diffracted light, andzeroth-order light) through a hologram element 16-11, which arecollimated by a collimator lens into convergent light. The aperturethrough which the convergent light passes is limited by an aperture film16-48 provided on a glass substrate of the aperture element 16-41according to the seventh embodiment of the present invention via themirror for bending an optical path, and then is incident through anobjective lens onto a signal surface of each of optical disks withvarious substrate thicknesses. The objective lens is designed to have ashape that enables aberration to be minimum by designing apertures andoptical systems optimally for respective disks having a substratethickness of 0.6 mm with respect to a beam with a wavelength of 660 nmand a substrate thickness of 1.2 mm with respect to a beam with awavelength of 780 nm. Light reflected from the signal surface passesthrough the aperture element 16-41. However, the aforementioned{fraction (5/4)} wave film is set for the beam with a wavelength of 660nm and therefore with respect to the wavelength of 780 nm, the filmbecomes an almost 1 wave film. Thus, the following conversion is notperformed: linearly polarized light→circularly polarized light→linearlypolarized light orthogonal to the incoming path. Consequently, nodiffraction is caused by the polarization hologram 16-42. The lightpassing though the element is incident onto the hologram 16-11 via themirror 16-7, the collimator lens 16-6, and the prism 16-5, is diffractedby the hologram 16-11 and then is incident onto detection surfaces ofthe photodetectors 16-16.

Eighth Embodiment

FIG. 17 shows a cross-sectional configuration of an aperture element17-41 according to an eighth embodiment of the present invention. Theelement is formed by sandwiching a polarization hologram layer and awave film between two glass substrates. On the glass substrate 17-46, aconcave-convex structure 17-43 is formed of a birefringent material(with refractive indexes of n1, n2) such as liquid crystal, and anisotropic adhesion layer (with a refractive index of n1) 17-44 isprovided thereon. A film 17-45 made of birefringent resin that isprovided adjacently to the adhesion layer 17-44 has an optical thicknesscorresponding to {fraction (5/4)} wavelengths with respect to a beamwith a wavelength λ1 (for example, 660 nm) with its refractive index,thickness, birefringence orientation with respect to a polarizationdirection being optimized. The optical thickness mentioned abovecorresponds to almost 1 wavelength with respect to a beam with awavelength λ2 (for example, 790 nm) emitted from a light source.Therefore, linearly polarized light passes through the above-mentionedpolarization hologram layer without being diffracted only with respectto the beam with the wavelength λ1, and the light that has beenreflected by a reflection surface and is incident from the oppositedirection is totally diffracted by the polarization hologram layer. Onthe other hand, with respect to the beam with the wavelength λ2, a planeof polarization is not varied and therefore the beam with the wavelengthλ2 is not diffracted even when passing through the element both in theincoming and return paths. Generally, this effect can be obtained whenwavelengths λ1 and λ2 of the two kinds of beams passing through theelement satisfy the relationship of (N1+¼) λ1≈N2×λ2 (wherein N1, N2=1,2, 3 . . . ).

On the other glass plate 17-47, a color separation film 17-48 thattransmits the beam with the wavelength λ1 and shields the beam with thewavelength λ2 is formed. Furthermore, a phase adjustment film 17-49 forcompensating the phase difference between lights passing through regionsA and B is formed on the glass plate 17-47. Thus, the beam with thewavelength λ1 passes through both the regions A and B, and the beam withthe wavelength λ2 passes through only the region B. In other words, theaperture is restricted. Furthermore, on the opposite surface to thesurface of the glass plate 17-46 on which a diffraction grating isformed, a concentric stepped structure 17-50 has been formed beforehandand has a function of compensating chromatic aberration caused by shiftin wavelength of a beam emitted form a laser beam source 1 as describedin the conventional example. In addition, since the beam with the otherwavelength λ2 is originally of almost spherical wave, the optical systemis required to be set so as to compensate the spherical wave.

The element 17-41 is formed by sandwiching the wave film 17-45 and thediffraction grating portion 17-43 between these glass substrates 17-46and 17-47 via the adhesion layers 17-44 and 17-51. In this case, sincethe element can be formed in wafer unit in a manner of mask alignment ina semiconductor process, the relative position of the centers of anaperture limitation film 17-48, a diffraction grating 17-43, and theconcentric stepped configuration 17-50 is not shifted, In addition, goodmass-productiveness is obtained.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof The embodiments disclosed inthis application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. An optical information processor performing atleast either one of information recording and information reproductionwith respect to an information recording medium, comprising: a lightsource; an objective lens for focusing light emitted from the lightsource on the information recording medium; a separation element forseparating light from the information recording medium from an opticalpath to the light source; first photodetectors for receiving lightseparated by the separation element, wherein an aperture NA1 in a firstoptical path from the light source to the information recording mediumand an aperture NA2 in a second optical path from the informationrecording medium to the first photodetectors are formed so as to satisfya relationship of NA1>NA2, wherein the aperture in the second opticalpath is formed of an aperture element comprising a polarization hologramportion and a thin film structure, the polarization hologram portion isformed by sandwiching a diffraction grating made of a birefringentmaterial and a wave film having an optical thickness of (N+¼) λ1(wherein N indicates an arbitrary natural number) between two glasssubstrates, the thin film structure is attached to either one of theglass substrates and varies an aperture area respectively for two lightswith wavelengths λ1 and λ2 (λ1<λ2) passing through the aperture element.2. The optical information processor according to claim 1, wherein theother glass substrate, to which the thin film structure is not attached,of the two glass substrates (with a refractive index ng) is providedwith a structure having a plurality of concentric stepped portions inwhich difference in height between adjacent stepped portions isλ1/(ng−1).
 3. The optical information processor according to claim 1,wherein the wavelengths λ1 and λ2 of two kinds of lights passing throughthe aperture element satisfy a relationship of (N1+¼)λ1≈N2×λ2, whereinN1 and N2 represent arbitrary natural numbers.
 4. An optical informationprocessor performing at least either one of information recording andinformation reproduction with respect to an information recordingmedium, comprising: a light source; an objective lens for focusing lightomitted from the light source on the information recording medium; aseparation element for separating light from the information recordingmedium from an optical path to the light source; wherein an aperture NA1in a first optical path from the light source to the informationrecording medium and an aperture NA2 in a second optical path from theinformation recording medium to the first photodetectors are formed soas to satisfy a relationship of NA1>NA2, wherein the aperture in thesecond optical path is formed of an aperture element comprising apolarization hologram portion and a thin film structure, thepolarization hologram portion is formed by sandwiching a diffractiongrating made of a birefringent material and a wave film having anoptical thickness of (N+⅕)λ1 (wherein N indicates an arbitrary naturalnumber) between two glass substrates, the thin film structure isattached to either one of the glass substrates and varies an aperturearea respectively for two lights with wavelengths λ1 and λ2 (λ1<λ2)passing through the aperture element.
 5. The optical informationprocessor according to claim 4, wherein the other glass substrate, towhich the thin film structure is not attached, of the two glasssubstrates (with a refractive index ng) is provided with a structurehaving a plurality of concentric stepped portions in which difference inheight between adjacent stepped portions is λ1/(ng−1).
 6. The opticalinformation processor according to claim 4, wherein the wavelengths λ1and λ2 of two kinds of lights passing through the aperture elementsatisfy a relationship of (N1+⅕)≈N2×λ2, wherein N1 and N2 representarbitrary natural numbers.